1. Field of the Invention
This invention relates to internal combustion engines which employ flexible link transmissions, such as belts. The belts rely on mechanical adherence, or friction, to drive a pulley. Such a belt operates driven devices which receive mechanical power via the belt which is driven by the internal combustion engine. The belts are used, more specifically, to drive accessories on automobile engines. The belts must be placed under a specific amount of tension for proper operation, and this tension may be applied by a roller positioned at the end of a movable lever arm. The lever arm is generally a part of a tensioning device which receives an elastic torque that can be provided by biasing means. The tensioning device elastically presses the roller, or a pulley against the flexible link, which may be a flat, trapezoidal or multi-ribbed belt.
2. Background Information
Flexible link transmissions which operate by mechanical adherence of a flexible link to pulleys, require an elastic tensioning means to make possible the mechanical adherence of the flexible link to the pulleys. This is essentially true whether the driving surface of the flexible link is flat, trapezoidal or grooved. Such a flexible link is commonly called a "belt", and these belts are generally made of cabled materials having high rigidity, such as polyester, glass fiber or aramid.
Such flexible links, or belts are used on automobiles, or stationary engines to drive accessories, such as water pumps, alternators, air conditioners, power steering pumps, etc. These accessories are positively driven by the flexible link which connects the accessories to a driven shaft of the engine. The adherence of the belt to the pulleys of the driven accessories and to the driven shaft is generally maintained by a tensioning device which utilizes an idler pulley.
The tensioning devices of flexible transmission links of the prior art exert an elastic thrust on the transmission belts which operate by mechanical adherence, or on the chain of a transmission. This elastic thrust is generally exerted by means of a movable idler pulley supported in general by an oscillating arm on which an elastic recall torque is exerted by a spring. In chain or positive drive devices, e.g. a so-called synchronized flexible belt, the device called the tensioning roller is generally blocked, after adjustment, to prevent dynamic pulsations in the strands of the flexible link between the pulleys.
The same is essentially true, for other reasons, in flexible link transmissions such as trapezoidal belts, when the elasticity of the flexible armature is used to provide an appropriate elastic tension between pulleys with a fixed center-to-center distance, as disclosed in European Patent No. 0,281,432 (Hutchinson).
The term "idler pulley", which is now in common usage, is applied, strictly speaking, to the recent generation of so-called multi-ribbed flexible belts, which are frequently used in the automobile industry. For these multi-ribbed belt systems, mechanical adherence is achieved by a plurality of inclined surfaces on the ribbed throats of the pulleys. The very high longitudinal rigidity of the flexible, textile reinforcement armature is advantageously offset by an elastic force exerted by a spring which applies a torque to the arm supporting the idler pulley. It is this spring that is deflected when there are dynamic variations of the tension.
In a first family of such idler pulleys, a bearing, which supports the shaft of the lever, receives an elastic torque from a metal spring. The metal spring, generally called a torsion spring which can be either a coil or a spiral spring, makes possible a high degree of flexibility, thus guaranteeing the application of an essentially constant force in spite of the dynamic variations, as disclosed in French Patent No. 2,509,408 (Litens Automotive) or U.S. Pat. No. 4,472,162 (Dyneer Corporation). The high degree of flexibility achieved by the above combinations results in a need to seek a damping of the dynamic movements by the friction of rubber rings. The rubber rings described are cylindrical in the former document and conical in the latter. The problems encountered in developing such a system generally arise due to the definition of a surface friction of rubber against metal during the life of an automobile.
European Patent No. 0,013,704 (Boge) discloses a device which belongs to the above-mentioned family by the utilization of a metal leaf spring which is coiled into a spiral. The device of European Patent No. 0,013,704 is also related to the following family of idler pulleys on account of the adherance of an elastomeric damper to the coiled spring.
A second family of idler pulley devices overcomes the risks of friction by exerting the elastic torque by the rubber itself, which is adherized to metal rings.
U.S. Pat. Nos. 3,975,965 (Dayco Corporation), 4,144,772 (The Toro Company), Great Britain Patent No. 2,070,727 (Alan Crosley Pritchard), and European Patent No. 0,157,193 (RIV-SKF) describe this type of device. The first, U.S. Pat. No. 3,975,965, proposes an improvement of the connection with the shaft by means of fin-like projections which increase the contact surface between the shaft and the elastomer. The remaining three patents propose improvements by using a square shaft. In each of these four cases, the rotation of the arm supporting the pulley takes place by deformation of the elastomer. Although the flexibility of the elastomer makes possible the torsional flexibility required by the tensioning device, it also results in imprecise guidance in the plane of the arm, which is not without certain disadvantages, in particular with regard to vibrations.
The damping of pulsation phenomena also results from the visco-elasticity of the material. This damping is frequently insufficient for certain resonant conditions of the engine. Thus, complex means have been developed to remedy the problem, such as a band brake, controlled electronically, as disclosed in U.S. Pat. No. 4,702,727 (Daimler-Benz), or a viscous fluid damping, as disclosed in PCT application No. WO 84/01415 (Dayco Corporation).
Non-torsional springs can also be used on a tipping device by rotation on a bearing, such as the elastic strap combined with a variable friction cam effect as disclosed in U.S. Pat. No. 4,571,223 (General Motors), or even a spring with a rectilinear coil embedded in an elastomer sheath which forms a chamber for a damping liquid as proposed in European Patent No. 0,243,237 (Hutchinson). French Patent No. 2,540,954 (Porsche) discloses, in addition to a stack of conical washers forming a rectilinear spring, an electrical control of the tension.
Such sophisticated measures, added to what was initially a very simple elastic tensioning system, are proof that the problem of maintaining the tension of the flexible link cannot be solved by means of only a simple spring. The experience of manufacturers of idler pulleys shows that the dynamic problems of resonance under certain engine conditions are extremely difficult to solve by the simple addition of exterior, visco-elastic or friction damping.
An analysis of the prior art shows that apparently, it does not include a simple, and economical elastic tensioning device for flexible link transmissions, which device includes an integrated means for damping the dynamic variations of tension, which damping, since it does not employ any friction element, minimizes the risks of variation over time and premature wear, and which device makes possible a simple regulation of a constant tension.